There are many systems in which movement of a tool or other device must be precisely controlled in accordance with predetermined parameters. The machine tool industry is a prime example; drilling and tapping machines, shapers, hobbers, mills, and rotary cutoff machines all require precision control of the movements and positioning of their machining devices. Similar requirements are encountered in conveyors and material handling installations, in pulling equipment, and in various forms of production equipment (e.g., molding and extrusion). Indeed, the same basic control requirements may occur in such diverse environments as apparatus for precision positioning of a telescope or the equipment for guiding a floor sweeper.
Conventional precision controls for such systems usually compute or calculate a signal representative of the desired movement to be carried out by a tool or other controlled device. This signal, usually called an error signal, is supplied to a motor or other prime mover to impel the device over the desired route to its destination. At regular intervals the progress of the device is monitored; corrections in the error signal are computed and made as needed. This kind of control can be quite accurate and precise, but it is subject to many problems and difficulties, including aging and thermal drift in amplifiers and other operating circuits, changing load conditions, unanticipated shocks or abrupt discontinuities in the motions of the controlled device, etc. As a consequence, for precision operation a conventional system may and usually does require tight specifications and adjustments during all stages, including installation, programming, design and servicing for both the control and the associated components.